resistance. However, the added drag of locked propellers may be desirable for some hull forms for better directional
b. Controllable-pitch propellers may be left installed if set in "maximum forward" pitch, where they offer the least
resistance to towing.
c. When propellers must be allowed to free-wheel, propulsion machinery must be disconnected from the shafts or
The procedure of free-wheeling propellers is not recommended.
Do not allow main reduction gears to rotate unless they are properly lubricated. This requires full lube oil
d A means for lubricating the shaft bearings must be provided. The stern gland on the shaft will normally be water-
lubricated. Provision for this must be made while at the same time ensuring that the water does not flood the space.
e When propellers remain in place and are not allowed to free-wheel, the shafts must be locked by an installed shaft-
locking device or by another suitable method as illustrated in Figure 3-3.
3-2.7.3 Trim. Proper trim of the tow is important because the trim can affect stability, towing characteristics and speed
through the water. Ballast, fuel, cargo or equipment on board may be shifted to effect the desired trim. Trimming by the
stern has proven an effective measure; a trim of one foot by the stern for each 100 feet of the tow's length has proven a
good trimming rule, deep draft tows use somewhat less than one foot per 100 feet. When liquid is used to trim the
vessel, care must be taken to ensure that there is no adverse free-surface effect. All tanks should be pressed full or left
empty. All sluice valves should be closed In the case of landing craft and other craft with blunt or raked bows, ballasting
may prevent heavy pounding, which can be very destructive to the vessel's bottom and other structural members.
Prevention or reduction of pounding also reduces shock loads on the towing rig The tow should have a zero list.
3-2.7.4 Stability. Stability of the tow, in the case of an unmodified or undamaged. Navy commissioned ship, can be
determined from a review of the data in Chapter II(a) of the ship's Damage Control Book. Similar information for
commercial ships should be available in the ship's Trim and Stability Booklet as well as in the Deadweight Survey. For
circumstances where formal documentation of the ship's stability is not available, the stability may be approximated by
timing the ship's roll period. This method is reasonably accurate and is used by the U.S. Navy, U.S. Coast Guard and
regulatory bodies to confirm the accuracy of inclining experiments and other similar stability determinations. For small
craft, timing the period of roll is the approved method of stability determination.
This roll period estimate can be performed accurately enough even in fairly calm water by watching the masthead. Time
several successive rolls and divide the total time by the number of rolls observed to obtain a good estimate. Then, to
determine the adequacy of the roll stability, compare the timed period with the value calculated from the following
For adequate stability, the time in seconds for a ship to roll from port to starboard and back to port must be equal to or
less than the time (T) calculated. For example, for a ship with a beam of 100 feet, the time observed for the ship to
complete a roll period must be less than the 20 seconds calculated. If the observed time is longer than the calculated
value (T), the